Re: [Xen-devel] Re: how to handle paged hypercall args?

[Olaf Hering <olaf@xxxxxxxxx>]

On Thu, Dec 02, Keir Fraser wrote:

> On 02/12/2010 10:22, "Keir Fraser" <keir@xxxxxxx> wrote:
> 
> > Firstly, the usage of waitqueues is broken. The waitqueue_head should be
> > shared with code that pages in, so that vcpus can be *woken* at some point
> > after they start waiting. As it is, if a vcpu does sleep on its local
> > waitqueue_head, it will never wake. You might start with a single global
> > waitqueue_head and wake everyone on it every time a page (or maybe page
> > batch) is paged in. More sophisticated might be to hash page numbers into a
> > array of waitqueue_heads, to reduce false wakeups.
> 
> ...Or you might have a per-domain waitqueue_head, and do the wake_up() from
> code that adds paged-in entries to the guest physmap. That would seem a
> pretty sensible way to proceed, to me.

Thats what I'm doing right now.

It seems that the existing MEM_EVENT_FLAG_VCPU_PAUSED code can be reused
for this. I was messing with wait_event() until I realized that the vcpu
is stopped by p2m_mem_paging_populate() already and the wake_up() ran
before the vcpu got a chance to call schedule().

If a vcpu happens to be scheduled and the domain is destroyed, the
BUG_ON in destroy_waitqueue_vcpu() will trigger. What can happen if
there is still an entry in the list? The cleanup should handle this
situation to not crash Xen itself.

Olaf


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